Method of forming a panel from a metal alloy sheet

ABSTRACT

A method is provided for forming a panel from a metal alloy sheet having known stress-strain forming properties including a forming strain limit that is exceeded when a single forming operation is used to form the panel. The method comprises determining a stress relieving treatment and an incremental forming treatment for forming the panel. The total incremental forming strain comprises a first strain increment. The stress relieving treatment results in the total incremental forming strain remaining within the forming strain limit. A first preform shape is determined in which the geometry of the shape does not exceed the first strain increment. The sheet is formed into the first preform shape. The first preform shape is then stress relieved. The first preform shape is then formed into a panel shape having a geometry that is within the total incremental forming strain.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to incrementally forming a panel from a strain hardenable metal alloy sheet that has a forming strain limit that may be exceeded if the panel is formed in a single forming operation.

2. Background Art

Styling is an important consideration in designing a vehicle that may be limited by the manufacturing feasibility of producing body panel parts with complex shapes. Often the panels are made of metal alloy sheets. Two examples of commonly used production forming methods for these panels are metal sheet stamping and more recently, hydroforming. Single sheet forming operations, such as for example, progressive die stamping of a panel, may not be feasible in producing more complex shapes. Body panels having complex shapes may have deep draw geometries that exceed the draw limitations of the forming operation. The ability to form complex shapes in body panels is compounded by the fact that the metal alloy sheets may be comprised of high strength steel or aluminum alloys that have less formability than traditional mild steels.

Two solutions employed today to circumvent this issue are to either limit the complexity of the geometry or produce smaller panels which may be welded or riveted in order to create a larger panel with more complex geometries. Both of these solutions either add cost and complexity to the manufacturing process, or limit design flexibility.

Accordingly, there is a need for an enhanced deep draw forming methodology for metal alloy sheets that have limited formability.

SUMMARY OF THE INVENTION

In one embodiment of the present invention a method of forming a panel from a metal alloy sheet is provided. The metal alloy sheet has a grain microstructure and known stress-strain forming properties. Two of such properties may include a monotonic ultimate elongation and a forming strain limit. The forming strain limit of the sheet may be exceeded at a particular location in the panel when a single forming operation is used to shape the sheet into the panel. The sheet is strain hardenable and is initially provided in a tempered condition for forming. The method comprises determining a stress relieving treatment for forming the panel that does not generate substantial grain growth of the grain microstructure. An incremental forming treatment is determined that includes a total incremental forming strain that is initially formed to a first strain increment. After the first strain increment the panel is stress relieved. The stress relieving treatment enables the total incremental forming strain to remain within the forming strain limit of the sheet. A first preform shape is determined as a forming precursor of the panel. The geometry of the first preform shape does not exceed the first strain increment. The sheet is formed into the first preform shape. A stress relieving treatment is provided to at least a portion of the first preform shape. The stress relieved first preform shape is formed into the panel by the incremental forming treatment, wherein the geometry of the panel does not exceed the total incremental forming strain and is within the forming strain limit of the sheet.

In at least one other embodiment of the present invention a method of forming a panel from a metal alloy sheet is provided. The metal alloy sheet has a grain microstructure and known stress-strain forming properties. Two of such properties may include a monotonic ultimate elongation and a forming strain limit. The forming strain limit of the sheet may be exceeded at a particular location in the panel when a single forming operation is used to shape the sheet into the panel. The sheet is strain hardenable and is initially provided in a tempered condition for forming. The method comprises forming the sheet into a first preform shape by an incremental forming treatment, wherein the incremental forming treatment includes a total incremental forming strain which exceeds the monotonic elongation. The total incremental forming strain further comprises a first strain increment. The first preform shape is a forming precursor of the panel and the geometry of the first preform shape does not exceed the first strain increment. At least a portion of the first preform shape is stress relieved by a stress relieving treatment, wherein the stress relieving treatment does not substantially recrystalize the grain microstructure and enables the total incremental forming strain to be within the forming strain limit of the sheet. The stress relieved first preform shape is formed into the panel by the incremental forming treatment, wherein the geometry of the panel does not exceed the total incremental forming strain and is within the forming strain limit of the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial perspective view of a vehicle having a body panel with a shape that has a deep draw geometry;

FIG. 2 is a sectional view of a sheet metal forming operation performed by stamping;

FIG. 3 is a side view of a driver side fender where the deep draw geometry of the fender has been damaged by the forming operation;

FIG. 4 is a stress-strain diagram of one embodiment of an incremental forming process for a metal alloy sheet;

FIG. 5 is a diagram of incremental forming results for 6016-T4 aluminum alloy of the final elongation and average drop in yield strength as a function of temperature using strain increments of 12%+6%+6%+ . . . etc., in accordance with one embodiment of the present invention;

FIG. 6 is a diagram of incremental forming results for 6111-T4 aluminum alloy of the final elongation and average drop in yield strength as a function of temperature using strain increments of 12%+4%+4%+ . . . etc., in accordance with one embodiment of the present invention;

FIG. 7 a is a photo of the grain microstructure of 6111-T4 after 12% strain and no heat treatment in accordance with one embodiment of the present invention;

FIG. 7 b is a photo of the grain microstructure of 6111-T4 after 12%+4%+4%+4% strain and 4 heat treatments in accordance with one embodiment of the present invention;

FIG. 7 c is a photo of the grain microstructure of 6111-T4 after 12%+4%+4%+4%+4%+4%+4%+4% strain and 8 heat treatments in accordance with one embodiment of the present invention;

FIG. 8 a is a photo of the grain microstructure of 6016-T4 after 12% strain and no heat treatment in accordance with one embodiment of the present invention;

FIG. 8 b is a photo of the grain microstructure of 6016-T4 after 6%+4%+4%+4%+4%+4% strain and 6 heat treatments in accordance with one embodiment of the present invention;

FIG. 8 c is a photo of the grain microstructure of 6016-T4 after 12%+4%+4%+4%+4%+4%+4%+4%+4%+4%+4%+4% strain and 12 heat treatments in accordance with one embodiment of the present invention;

FIG. 9 is a diagram of the incremental forming results for 6016-T4 with heat treatments of 30 seconds in a peak specimen temperature of 110 C in accordance with one embodiment of the present invention;

FIG. 10 is a diagram of the incremental forming results and the effect of duration of heat treatment on the finally elongation of 6016-T4 and 6111-T4 in accordance with one embodiment of the present invention;

FIG. 11 is a diagram of incremental forming results for 5754-O aluminum alloy of the final elongation and average drop in yield strength as a function of temperature using strain increments of 10%+10%+ . . . etc., in accordance with one embodiment of the present invention;

FIG. 12 a is a photo of the grain microstructure of 5754-O pre-stained 17% and heated at 400 C with contact heating for 30 seconds;

FIG. 12 b is a photo of the grain microstructure of 5754-O pre-stained 17% and heated at 400 C with contact heating for 40 seconds;

FIG. 12 c is a photo of the grain microstructure of 5754-O pre-stained 17% and heated at 400 C with contact heating for 60 seconds;

FIG. 13 a is a photo of a grain microstructure of 5754-O, 17% pre-stain and heated at 600 C with contact heating for 8 seconds;

FIG. 13 b is a photo of a grain microstructure of 5754-O, 17% pre-stain and heated at 600 C with contact heating for 10 seconds;

FIG. 13 c is a photo of a grain microstructure of 5754-O, 17% pre-stain and heated at 600 C with contact heating for 12 seconds;

FIG. 13 d is a photo of the grain microstructure of 5754-O with an elongated grain structure after six strain increments of 10%, and 5 heat treatments of the peak temperature of 375 C in a duration of 30 seconds; and

FIG. 14 is a flow chart illustrating an embodiment of the method of forming a panel from a metal alloy sheet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Detailed embodiments of the present invention are disclosed herein. It is understood, however, that the disclosed embodiments are merely exemplary of the invention and may be embodied in various and alternative forms. The figures are not necessarily to scale, some figures may be exaggerated or minimized to show the details of the particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for the claims or as a representative basis for teaching one skilled in the art to practice the present invention.

Referring to FIG. 1, a partial perspective view is provided of a vehicle 10. The vehicle 10 includes a fender body panel 12, an exterior door trim panel 14, an exterior lighting lens 16, and a bumper fascia 18. The exterior lens 16 and the bumper fascia 18 are produced by injection molding a plastic material. Other panels, such as the fender 12, are often manufactured by stamping or hydroforming a metal alloy sheet. Styling requirements may dictate highly contoured geometric shapes that may exceed the forming strain limit in single stamping or hydroforming operation.

In FIG. 2, a sectional view is provided of a sheet metal stamping operation 30. The stamping operation 30 represents one of several ways of forming sheet metal into a panel. Another method of forming sheet metal into a panel is by hydroforming. A metal alloy sheet 32 is forced into a cavity 34 by the core 36 of the tool or die in the stamping operation 30. Clamping pads 37 with regular springs or gas springs may be used to clamp the sheet 32 to help prevent wrinkling of the sheet 32 during stamping. The cavity 34 had a deep draw volume 38 which is used to form a deep draw shape within the metal alloy sheet 32. Conventional stamping operations may have multiple stages using multiple dies for forming. One example of a conventional stamping operation is a progressive die stamping operation. In this operation, the metal alloy sheet 32 may be stamped several times where each stamping performs a unique forming step. The process of stamping and forming the metal sheet 32 multiple times may result in accumulating dislocations and residual stress as a result of non-uniform distribution of plastic deformation between material grains. This factor may limit the sheet's 32 ability to be stretched or drawn. Stated another way, strain hardening effectively reduces the forming strain limit of the sheet 32. When this occurs, the metal sheet 32 is less able to be properly formed into the deep draw volume 38 of the cavity 34.

In FIG. 3, a side view is provided of the driver side fender 12 with deep draw geometry. The fender 12 has a deep draw area located in the nose 50 of the fender 12 that interfaces with the headlamp 16. As shown, the nose 50 has a crack 54 that resulted from the forming operation. A crack 54 may sometimes only occasionally occur in high volume production of a highly contoured panel. The nose 50, for example, may be susceptible to thinning out of the material, which may also occasionally result in cracking.

The present invention provides a method of forming a panel from a metal alloy sheet that has a forming strain limit that is susceptible to strain hardening. The forming strain limit of the sheet may be exceeded at a location in the panel in a single forming operation. The forming capability of a metal alloy sheet may be enhanced by using incremental forming at respective strain increments interposed with stress relieving treatments. The following study performed by the Applicants illustrates at least one embodiment of the present invention.

The materials used in this study are shown in Tables 1 and 2 with their respective mechanical properties and chemical compositions.

TABLE 1 Gauge Y.S. U.T.S. Alloy Manufacturer (mm) (MPa) (MPa) % EL* 6016-T4 EDT Alcan 1.02 116 230 27.9 6111-T4 PD Alcan 0.93 145 284 26.8 5754-O Alcoa 2.00 115 220 25.0 *Monotonic Ultimate Elongation

TABLE 2 6016-T4 EDT 6111-T4 PD 5754-O Mg 0.61 0.89 2.6–3.6 Si 0.97 0.54 <0.40 Cu 0.04 0.67 <0.10 Fe — 0.19 <0.40 Mn 0.04 0.22 <0.50 Al Bal. Bal. Bal.

Interrupted tensile testing was used to simulate the incremental forming process. Tensile specimens were cut from aluminum sheet stock. In testing, the tensile specimens were deformed to a specific amount of strain, then removed from the load frame and heat treated by various methods to promote recovery and stress relief, and then placed back into the load frame for further deformation. This procedure was then repeated. The specimens were put through these strain/heat treatment increments until they ultimately failed, and the final elongation was measured and used as an indication of the effectiveness of the heat treatment parameters. The stress-strain data for each deformation increment was recorded, and with this information it was possible to measure the response of the material to the heat treatment in terms of strength reduction, or softening. More specifically, the quantity measured was the difference between the peak stress achieved by a specimen at the end of a strain increment, and the yield stress of that same specimen after being heat treated.

FIG. 4 is a schematic of the incremental forming process and the drop in yield stress, which occurs in response to an intermediate heat treatment as illustrated in the figure as Δσ.

Microstructural analysis was conducted using an optical microscope. Samples of each alloy were exposed to various amounts of accumulated strain in different heat treatment conditions, and the resulting microstructures were analyzed in order to determine if any recrystallization or grain growth had occurred.

The results of the study for the 6xxx series aluminum alloys is discussed first. It was originally found for 6111-T4 that a heat treatment of 30 seconds in a furnace maintained at 250 C resulted in excellent formability for this alloy. When using strain increments of 12%, followed by 4% in each increment after the first, the elongation increased from 27% in the as received condition, otherwise referred to as the monotonic ultimate elongation, to 45% with incremental forming. These same conditions were chosen as a starting point for experimentation with 6016-T4, and the result was an increase in elongation from the 28% monotonic ultimate elongation to 65% elongation with incremental forming.

Further testing was conducted with both the 6016-T4 and the 6111-T4 alloys. Using a tensile specimen with an attached thermocouple, it was found that the specimens reached a temperature of 110±5 C after being heated in a tray for 30 seconds in a furnace at 250C. In order to measure the effect of the peak specimen temperature on 6016-T4, tests were run using the same 30 second heat treatment duration, but with peak specimen temperatures of 90 C, 110 C, 130 C, 150 C and 170 C. The strain increments used in these tests were 12% in the first increment, followed by 6% in each increment after the first, and the results are shown in FIG. 5.

FIG. 5 illustrates that a 30 second heat treatment with a peak specimen temperature in the range of 110 C to 150 C results in a significant increase in the formability of 6016-T4. The results indicate that heating the specimen to a peak temperature of 90 C did promote some recovery and stress relief, but not nearly as much as the peak temperatures in the range of 110 to 150 C. Heating the specimen to a peak temperature of 170 C also promoted some recovery and stress relief. The fact that the results were much lower than those at 110 C to 150 C indicates that artificial aging is taking place along with recovery and stress relief at the higher temperatures. An overall decrease in material formability were caused relative to the results at 110 C to 150 C. FIG. 5 also illustrates that the average drop in yield stress follows a trend similar to that of the elongation.

The same conditions were then applied to 6111-T4, and the results are shown in FIG. 6. It should be noted, that the strain increments used in this experiment, however, were 12% followed by increments of 4%.

FIG. 6 illustrates that 6111-T4 responded similar to 6016-T4 in that there is a window of specimen peak temperatures within which incremental forming is significantly beneficial. Under these conditions, the 6016-T4 alloy's optimal temperature range was about 110 C to 150 C, and for the 6111-T4 alloy, this range was about 110 C to 130 C. This difference may be due to the higher Cu content of 6111-T4 relative to 6016-T4, which is known to increase precipitation hardening kinetics. As in FIG. 5, FIG. 6 shows a strong correlation between the final elongation and the drop in yield stress after heat treatment. The magnitude of the drop in yield stress of 6111-T4 is about half that of 6016-T4, with the value becoming negative at 170 C in the case of 6111-T4.

The microstructures of the 6111-T4 and 6016-T4 specimens were analyzed after undergoing numerous increments of deformation followed by heat treatments. Some representative microstructures resulting from the incremental forming of 6111-T4 and 6016-T4 are shown in FIGS. 7 and 8, respectively. In all cases, substantial recrystallization or grain growth were not observed. This result was obtained because for most aluminum alloys, recrystallization and grain growth usually occur only at temperatures greater than about 345 C. Because neither recrystallization nor grain growth were observed, the primary softening mechanism that was occurring during the intermediate heat treatments of 6xxx alloys is the relief of residual stresses, which occurs through redistribution of those stresses from isolated grains in the entire grain structure.

Further testing was conducted to evaluate the effect of strain increment size. Tests were run on 6016-T4 with a common heat treatment condition (30 seconds, peak temperature of 110 C), with the following strain schedules (%): 12+4+4+ etc., and 12+6+6+etc., and 12+12+ etc. The results of these tests are shown in FIG. 9. As illustrated, even with larger strain increments and thus fewer heat treatments for stress relief, incremental forming still provides a benefit to the overall forming elongation.

A typical production rate for automotive body panels with conventional stamping processes is approximately 300 panels per hour, or 1 panel per 12 seconds. A heat treatment time of 30 seconds between forming increments on a stamping line may limit production output. The duration of the heat treatments may be fit within the 12 second window preferred by conventional stamping rates for incremental forming. The heat treatments may also be performed using off-line batch processing to provide a greater time window.

Testing was further conducted with tensile specimens that were heated more rapidly to an operative temperature range of 110 C to 170 C and within a time period range of 4 to 8 seconds, depending upon the test. Starting with 6016-T4, specimens reached a peak temperature of 165±10 C in 8 seconds. Using strain increments of 12%+4%+ . . . , 6016-T4 showed a final elongation of 60%. This value was almost as large as the 65% elongation achieved with a 30 second heat treatment at a peak specimen temperature of 110 C Tests were then run using a heat treatment duration of 4 seconds and a peak specimen temperature of 157±10 C. The resulting elongation of 6016-T4 with strain increments of 12+4+ . . . % was 52%. This value was also a significant increase over the monotonic ultimate elongation of 28%. These results are shown in FIG. 10.

FIG. 10 illustrates a time-sensitivity to the stress relief and recovery mechanism for 6016-T4 within a time range of 0-10 seconds. After about 10 seconds, the amount of additional stress relief and recovery that may be gained is minimal. There was virtually no difference between the elongation achieved with durations of 4, 8, and 30 seconds for 6111-T4. This difference may be due to the higher precipitation hardening kinetics of 6111-T4 relative to that of 6016-T4.

Next, the 5xxx series aluminum alloys were studied. Original testing on 5754-O including a heat treatment of 120 seconds in a furnace maintained at 600 C resulted in excellent formability for this alloy. Using strain increments of 12%, the alloy achieved an average final elongation of 87%. This value exceeds the 65% elongation achieved by 6016-T4 when forming with smaller strain increments of 12%+4%. This is because much higher heat treatment temperatures may be used with 5xxx series alloys since precipitation hardening does not occur in 5xxx series alloys. Heat treatment duration of 120 seconds may be undesirable for production operations. As a result, contact heating of durations of 8 to 30 seconds were used for further testing of 5754-O.

Precipitation hardening does not occur in 5xxx alloys. Accordingly, there is not a limited temperature window between which final elongation would be maximized as there may be with the 6xxx series alloys. The 5xxx series alloys may use higher heat treatment temperatures that would likely result in higher total elongation. Incremental forming of 5754-O tests were run at different temperatures using contact heating for 8 seconds and strain increments of 10%. The results are shown in FIG. 11.

Grain growth may occur in aluminum alloys at temperatures in excess of 345 C, which may produce reduced strength and poor surface quality after forming due to orange peel. The 5754-O specimens incrementally formed and heat treated for 120 seconds at 600 C exhibited a very rough surface finish after forming, indicative of significant grain growth and orange peel. Experiments were conducted with tensile samples prestrained to 17% to determine when recrystallization or grain growth would occur in the contact heating treatment of 400 to 600 C for 4 to 30 seconds. The specimens were heat treated for various amounts of time, sectioned, mounted, and polished for microstructural analysis. Specimens were then heat treated using contact heating with aluminum plates maintained at 400 C for durations of 12 seconds, 16 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds and 60 seconds.

Microstructural analysis showed that no recrystallization or grain growth had occurred through 30 seconds, but at 40 seconds approximately half of the grains had grown to three or four times their original size, and by 60 seconds almost all the grains had grown to the same size. Micrographs of the resulting structures are shown in FIG. 12. FIG. 12 a is 5754-O prestrained 17% and contacted heated at 400 C for 30 seconds; FIG. 12 b is the same as 12 a except it has 40 seconds of contact heating; and FIG. 12 c is the same as 12 a except it has 60 seconds of contact heating.

Other prestrained specimens were heat treated using contact heating with plates maintained at 600 C for durations of 4 seconds, 6 seconds, 8 seconds, 10 seconds, 12 seconds, 16 seconds, 20 seconds and 30 seconds. Microstructural analysis showed that no recrystallization or grain growth had occurred through 8 seconds (see FIG. 13 a), but at 10 seconds (see FIG. 13 b) grain growth was partially complete, and by 12 seconds (see FIG. 13 c) all the grains had grown larger than their original size. No recrystallization was observed with the prestrain level of 17% at both the 400 C and 600 C conditions, and grain growth was observed to be a somewhat rapid at onset, but then significantly slower after all the grains had begun to grow. Representative micrographs showing the progress of grain growth are shown in FIG. 13. The elongated grain structure resulting from extensive incremental forming (six increments of 10% and 5 heat treatments of a peak temperature of 375 C and a duration of 30 seconds—see FIG. 13 d) is without recrystallization or grain growth.

After determining the times and temperatures at which grain growth occurs in 5754-O, incremental forming tests were conducted with proper heat treatment conditions to determine how much elongation could be achieved without significantly altering the grain size of the material. At 400 C, a duration of 30 seconds is just below the onset of grain growth, and at 600 C, a time of 8 seconds is just below the onset of grain growth. Temperature measurements showed that the use of contact heating with plates at 400 C resulted in the tensile specimens reaching a peak temperature of 375 C in 30 seconds. Using plates at 600 C brought the tensile specimens to a temperature of 465 C in 8 seconds. Incremental forming tests were run with these heat treatment parameters and with strain increments of 10%.

The heat treatment of 30 seconds using 400 C plates resulted in an average final elongation of 63%. Heat treatments for 8 seconds using 600 C plates resulted in an average elongation of 52%. Microstructural analysis was performed on fractured tensile specimens deformed at these conditions and no grain growth was observed.

Referring to FIG. 14, one embodiment of a method of forming a panel from a metal alloy sheet is illustrated in a flow chart. In FIG. 14, a method is provided for forming a panel from a metal alloy sheet having a grain microstructure and known stress-strain forming properties including a monotonic ultimate elongation and a forming strain limit. The forming strain limit of the sheet may be exceeded at a location in the panel when a single forming operation is used to shape the sheet into the panel. The sheet is strain hardenable and is initially in a tempered condition for forming.

The metal sheet made be comprised of, for example, a steel or aluminum alloy. For instance, low carbon steel alloys such as SAE 1008 or 1010 may be suitable steel alloy sheet materials. Alternatively, 6016-T4, 6111-T4 and 5754-O are examples of suitable aluminum alloy sheet materials. Other suitable sheet materials may also be used that are known to those skilled in the art.

The method comprises determining a stress relieving treatment 200 for the metal alloy sheet. The stress relieving treatment includes a temperature-time treatment. The time duration of the temperature treatment according to the invention is preferably at least, with increasing preference in the order given, 1, 2, 3, or 4 seconds and independently, primarily for degradation and/or economic reasons, preferably is not more than, with increasing preference in the order given, 30, 20, 15 or 12 seconds. The temperature treatment, which may be unique to the respective metal alloy used and may also be dependent on the specific time treatment chosen, heats the grain microstructure to a temperature which redistributes residual stresses from isolated grains into the entire grain structure without generating substantial grain growth or recrystallization of the grain microstructure.

The method further comprises determining an incremental forming treatment 202 including a total incremental forming strain comprising a first strain increment, wherein the stress relieving treatment enables the total incremental forming strain to be within the forming strain limit of the sheet. In at least one embodiment, the stress relieving treatment enables the total incremental forming strain to exceed the monotonic ultimate elongation of the metal alloy sheet. The total incremental forming strain may be comprised of a plurality of strain increments. In at least one embodiment, the strain increments are within the range of approximately 4% to 17%.

The method further comprises determining a first preform shape 204 as a forming precursor of the panel. The geometry of the first preform shape is such that it does not exceed the first strain increment, wherein the sheet geometry represents a base strain of zero. There are corresponding preform shapes for each of the strain increments in embodiments where the total incremental forming strain is comprised of a plurality of strain increments. Each respective preform shape is a forming precursor of the panel, wherein the geometry of the preform shape does not exceed the sum of the prior and corresponding strain increments.

Incremental forming may include limiting the method to only deeper draw areas of the panel. For instance, in FIG. 3, the nose 50 region of the fender 12 may be a local area where incremental forming may be used. More specifically, a plurality of preform shapes may be determined and limited to this specific area of the panel. Alternatively, preform shapes and incremental forming may include the entire panel. Any suitable area of the panel determined by those skilled in the art, including the entire panel, may benefit from various embodiments of the present invention.

The method further comprises forming the sheet into the first preform shape 206. Various methods of forming may be used including stamping and hydroforming. Although production forming may benefit substantially from embodiments of incremental forming, it is understood that prototype forming processes are also included as suitable forming methods.

The method further comprises stress relieving the first preform shape 208 by the stress relieving treatment. The stress relieving treatment may be limited to a portion of the first preform shape or may include the entire first preform shape. In one embodiment, only a portion of the first preform shape is heated by a heat source for a time period. After removal of the heat source, the heated portion cools rapidly by thermal conduction into the remaining cooler portion of the first preform shape.

Various heat sources may be employed for the stress relieving treatment. For example, induction heating, contact heating, electric heating or oven heating may be suitable methods of heating depending on the desired cycle time. Other suitable methods of heating known by those skilled in the art may also be used.

The method further comprises forming the stress relieved first preform shape into the panel 210, wherein the geometry of the panel does not exceed the total incremental forming strain and is within the forming strain limit of the sheet. Any suitable method of forming may be employed as was previously discussed. Moreover, forming the stress relieved first preform shape into the panel may include in one embodiment, forming a plurality of preform shapes by the incremental forming treatment interposed with stress relieving treatments.

While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims. 

1. A method of forming a panel from a metal alloy sheet having a grain microstructure and known stress-strain forming properties including a monotonic ultimate elongation and a forming strain limit, where the forming strain limit of the sheet is exceeded at a location in the panel when a single forming operation is used to shape the sheet into the panel, the sheet being strain hardenable and being initially in a tempered condition for forming, the method comprising: determining a stress relieving treatment for forming the panel such that the stress relieving treatment does not generate substantial grain growth of the grain microstructure; determining an incremental forming treatment including a total incremental forming strain comprising a first strain increment, wherein the stress relieving treatment enables the total incremental forming strain to be within the forming strain limit of the sheet; determining a first preform shape as a forming precursor of the panel, wherein the geometry of the first preform shape does not exceed the first strain increment; forming the sheet into the first preform shape; stress relieving at least a portion of the first preform shape by the stress relieving treatment; and forming the stress relieved first preform shape into the panel by the incremental forming treatment, wherein the geometry of the panel does not exceed the total incremental forming strain and is within the forming strain limit of the sheet.
 2. The method according to claim 1 wherein the total incremental forming strain exceeds the monotonic ultimate elongation.
 3. The method according to claim 1 wherein forming includes stamping.
 4. The method according to claim 1 wherein forming includes hydroforming.
 5. The method according to claim 1 wherein the total incremental forming strain further comprises a second strain increment and the method further comprising: determining a second preform shape as a forming precursor of the panel, wherein the geometry of the second preform shape does not exceed the sum of the first and the second strain increment; and the step of forming the stress relieved first preform shape includes: forming the stress relieved first preform shape into a second preform shape; stress relieving at least a portion of the second preform shape by the stress relieving treatment; and forming the stress relieved second preform shape into the panel by the incremental forming treatment.
 6. The method according to claim 1 wherein the stress relieving treatment includes a temperature-time treatment, wherein the time for the temperature treatment is in the range of approximately 1 to 30 seconds.
 7. The method according to claim 6 wherein the temperature treatment includes contact heating.
 8. The method according to claim 6 wherein the temperature treatment includes induction heating.
 9. The method according to claim 6 wherein the metal alloy sheet is aluminum alloy of the AA6xxx family and the temperature treatment does not heat the grain microstructure to a temperature in excess of 345C.
 10. The method according to claim 9 wherein the time for temperature treatment is in the range of approximately 4 to 8 seconds and the temperature treatment heats the grain microstructure to a temperature in the range of approximately 145 C to 165 C.
 11. The method according to claim 9 wherein the first strain increment is in the range of approximately 10% to 17%.
 12. The method according to claim 6 wherein the metal alloy sheet is aluminum alloy of the AA5xxx family and the temperature treatment heats the grain microstructure to a temperature in the range of approximately 375 C to 465 C.
 13. The method according to claim 12 wherein the time for temperature treatment is in the range of approximately 4 to 12 seconds and the first strain increment is in the range of approximately 10% to 17%.
 14. A method of forming a panel from a metal alloy sheet having a grain microstructure and known stress-strain forming properties including a monotonic ultimate elongation and a forming strain limit, where the forming strain limit of the sheet is exceeded at a location in the panel when a single forming operation is used to shape the sheet into the panel, the sheet being strain hardenable and being initially in a tempered condition for forming, the method comprising: forming the sheet into a first preform shape by an incremental forming treatment, wherein the incremental forming treatment includes a total incremental forming strain which exceeds the monotonic ultimate elongation and comprises a first strain increment, wherein the first preform shape is a forming precursor of the panel and the geometry of the first preform shape does not exceed the first strain increment; stress relieving at least a portion of the first preform shape by a stress relieving treatment, wherein the stress relieving treatment does not substantially recrystalize the grain microstructure and enables the total incremental forming strain to be within the forming strain limit of the sheet; and forming the stress relieved first preform shape into the panel by the incremental forming treatment, wherein the geometry of the panel does not exceed the total incremental forming strain and is within the forming strain limit of the sheet.
 15. The method according to claim 14 wherein the total incremental forming strain further comprises a second strain incremental and the step of forming the stress relieved first preform shape includes: forming the stress relieved first preform shape into a second preform shape, wherein the second preform shape is a forming precursor of the panel and the geometry of the second preform shape does not exceed the sum of the first and the second strain increment; stress relieving at least a portion of the second preform shape by the stress relieving treatment; and forming the stress relieved second preform shape into the panel by the incremental forming treatment.
 16. The method according to claim 14 wherein the total incremental forming strain comprises a plurality of strain increments within the range of approximately 4% to 17%.
 17. The method according to claim 14 wherein the stress relieving treatment includes a temperature-time treatment, wherein the time for the temperature treatment is in the range of approximately 1 to 12 seconds.
 18. The method according to claim 17 wherein the metal alloy sheet is aluminum alloy of the AA6xxx family and the temperature treatment heats the grain microstructure to a temperature in the range of approximately 145 C to 165 C.
 19. The method according to claim 17 wherein the metal alloy sheet is aluminum alloy of the AA5xxx family and the temperature treatment heats the grain microstructure to a temperature in the range of approximately 375 C to 465 C.
 20. The method according to claim 17 wherein the step of forming the panel includes stamping. 